[ng] maintcheck: some tweaks and fixlets

[ng] maintcheck: some tweaks and fixlets

* syntax-checks.mk (sc_no_brace_variable_expansions): Do not complain
about ${MAKEVAR} usages in comments: they might be legitimate.  This
change is not strictly required for mainline automake, but will help
in the Automake-NG branch.
* sc_tests_here_document_format: Relax a bit, so that usages like
"cout << "string" << endl;" in the test scripts (usages which can be
used in here documents defining C++ sources) are not flagged as uses
of bad delimiters for here documents.  This change is not strictly
required for mainline automake, but will help in the Automake-NG
branch, and will anyway be more future proof w.r.t. possible future
testsuite additions.
(sc_tests_Exit_not_exit): Do not whitelist lines containing $PERL
explicitly (that was only required for one test); instead ...
* t/ext2.sh: ... use "exit (1)" instead of "exit 1" in the perl
invocation of the guilty script.

Signed-off-by: Stefano Lattarini <stefano.lattarini@gmail.com>